A maskant has been developed for use during laser welding or drilling of components, particularly components with protective coatings. The maskant is particularly useful when performing laser welding operations on turbine blades with ceramic thermal barrier coatings. The maskant improves the yield when performing weld operations on certain components, thereby reducing rework, scrap, and associated costs.
Current configuration turbine blades receive ceramic thermal barrier coatings. The ceramic coatings are typically in the range of 0.003"-0.007" thick. Conventional weld repair procedures require the removal of any coatings from the base alloy immediately adjacent to the weld areas to avoid contamination of the weld and to minimize cracking. During welding of the tip of a gas turbine engine blade when the laser beam makes the initial passes around the blade there is "overhanging" laser light that passes down the sides of the airfoil. The overhanging laser light does not cause damage on many part configurations; however, when blades exhibit a step caused by the local removal of the thermal barrier coating there is a problem. The overhanging beam often strikes the edge of the ceramic. The ceramic material (generally yittria stabilized zirconia) has a tendency to preferentially absorb the laser light from a CO.sub.2 laser. The very low thermal conductivity of the ceramic does not allow the heat to dissipate readily. The ceramic then melts, causing subsequent melting and deep pitting of the base alloy, resulting in damage or destruction of the part being welded.
One solution is to remove the ceramic to a level where it will not melt when hit by the laser beam (because the beam is sufficiently defocused). Unfortunately, removing the ceramic to that level exceeds the maximum allowable amount of coating removal for certain types of repair. The laser beam cannot be positioned in such a way that it does not overhang the edge of the blade, or there will be incomplete fusion at the base of the weld.
Another solution is to physically mask the edge of the ceramic to protect it from the laser beam. The edge of the ceramic can not be readily protected by a hard mechanical mask, since the mask must fit the part perfectly in order to block the laser beam. Custom masks would have to be created for each blade to be welded in order to take into account slight variations in the contour of each individual piece. Production of such custom masks is prohibitively time consuming and expensive.